In copending application, Ser. No. 689,115, filed on May 24, 1976, there is disclosed an intervenous pumping system which includes a pump actuator assembly 46 in which a plunger moves into and out of a chamber having a portion thereof covered by a flexible member which is filled with a constant quantity of liquid. It has been found to be very difficult to control leaks between the plunger and the housing. Such leaks permit air to enter into the chamber and thereby degrade the accuracy of the pumping operation which is to be carried out by the pump actuator assembly. There is therefore, made for an improved pump actuator assembly which will overcome this disadvantage.